Design, synthesis and pharmacological evaluation of molecular probes targeting the dopamine D2 receptor

The dopamine D2 receptor (D2R) is a member of the GPCR superfamily, and is inextricably linked to a number of important disease states, including schizophrenia and Parkinson’s disease. This thesis focuses on the design and synthesis of novel compounds to probe emerging concepts in D2R structure and function, including allosterism, dimerization, biased agonism, bitopic ligands and bivalent ligands. Firstly, a recent report highlighted SB269652 as the first drug-like allosteric antagonist of the D2R. However SB269652 shares structural homology with a number of orthosteric D2R antagonists and partial agonists. To understand this mechanism of action, we designed and synthesized truncated derivatives of SB269652 with the aim of discovering purely allosteric and orthosteric fragments (Chapter 2). Utilizing this approach, we found that SB269652 binds in a bitopic (dual orthosteric/allosteric) mode within one protomer whilst allosterically modulating a second protomer as part of a dimeric (or higher order) complex. Chapter 2 has been submitted for publication in Nature Chemical Biology. In Chapter 3 we explore the current knowledge surrounding the principles of partial and biased agonism at GPCRs. We outline recent challenges and successes, with particular focus on relating ligand structure to function. Chapter 3 forms the basis of a review article planned for submission to the Journal of Medicinal Chemistry, and includes extensive focus on the D2R; a major theme of the thesis. To further our understandings of biased agonism, Chapter 4 focuses on the synthesis of novel D2R ligands able to fine-tune efficacy and biased agonism using subtle structural modifications based on the hit D2R partial agonist, MIPS1026. The findings imply that biased ligands interact with a secondary D2R binding site, and thus potentially represent the first D2R bitopic ligands. Chapter 4 has been prepared for submission to the Journal of Medicinal Chemistry. Finally, Chapter 5 (published as a review; Shonberg, J. et al. ChemMedChem 2011, 6 (6), 963-974.) and Chapter 6 focus on the synthesis of bivalent ligands to explore the structure and function of D2R dimers and higher order oligomeric complexes. In Chapter 6, bivalent ligands were designed based on the aporphine scaffold with variations in linker attachment position and spacer length. We found that at certain lengths, bivalent ligands exhibited full recovery of functional efficacy equivalent to the potent D2R agonist, apomorphine. Chapter 6 has been prepared for submission to ACS Medicinal Chemistry Letters. Overall, this thesis successfully uses novel molecular probes to further our understandings of the D2R structure and function. We show the ability to design D2R biased agonists, allosteric fragments, bitopic ligands, and bivalent ligands with various effects, and diverse implications on the structure and function of the D2R.